Define operator for built-in and class instance

Question

In Python you can override an operation for your class (say, addition) by defining __add__ . This will make it possible add class instance with other values/instances, but you can't add built-ins to instances:

foo = Foo()
bar = foo + 6 # Works
bar = 6 + foo # TypeError: unsupported operand type(s) for +: 'int' and 'Foo'

Is there any way to make enable this?

Answer 1

当实例在右侧时__radd__(self, other)必须定义方法__radd__(self, other)来覆盖运算符+ 。

Answer 2

You cannot override the + operator for integers. What you should do is to override the __radd__(self, other) function in the Foo class only . The self variable references a Foo instance, not an integer, and the other variable references the object on the left side of the + operator. When bar = 6 + foo is evaluated, the attempt to evaluate 6.__add__(foo) fails and then Python tries foo.__radd__(6) (reverse __add__ ). If you override __radd__ inside Foo , the reverse __add__ succeeds, and the evaluation of 6 + foo is the result of foo.__radd__(6) .

def __radd__(self, other):
   return self + other